Hi minoubinette and welcome to Aquarium Advice!
Look at your betta from above -- do his scales look like they are sticking out like a pinecone? This is called dropsy. If he looks like this, give him some antibiotics, like Kanacyn, Maracyn, or Jungle Fungus Eliminator. Unfortunately, I have not had any luck with my bettas if they get to this stage.
Hopefully your betta is just constipated. Some signs of this are a swollen belly right around the pelvic fins (the long skinny ones) and a string of poop hanging from him. To help the constipation, feed a pea. This will hopefully clear any blockage. I like to use frozen peas. They don't get mushy like canned peas. Fill a glass measuring cup about half full of water, and microwave a few peas, for 4 minutes on high. Let cool, and peel and discard the skin of the pea. Cut the pea into pieces the same size as the betta's normal food -- he should accept it that way. I just squeeze the pea between my fingernails to get little pieces. Don't feed any of his normal food for a few days. This won't hurt him and it will give the blockage a chance to go away. You should try to give your betta a pea like this once a week, to keep his digestive system healthy. I feed my bettas pellets and bloodworms Monday through Friday. On Saturday they get peas, and on Sunday I don't feed them at all. Skipping a day of feeding once a week is good for their digestive sytem too.
The little parasites were probably ich. Read the ich article on this site:
http://www.aquariumadvice.com/showquestion.php?faq=2&fldAuto=32
Have you seen any more spots? Try the heat method if you see more spots. Sometimes medicines don't clear ich up completely. The ich parasite is only vulnerable to meds in the free-swimming stage. Otherwise, they are protected in the cyst. If there are still some cysts in the gravel, the spots could appear again on the betta. Watch and try the heat treatment if they come back.
What size tank is your betta in? My bettas are in 5.5 gallon tanks, and I do a water change once a week. If your betta is in a bowl, you should do changes twice a week. (also, the heat treatment for ich is out if the betta is in a bowl.) In my experience, bettas in a bowl get stressed easily, which leads to conditions like ich. Do you have test kits, to test the water parameters?
